This is a TD (Tower/Turret Defense) game, but the battlefield is the Windows desktop, the enemies are malware, the turrets are anti malware programs and the things you protect are your common Windows applications. Select the anti malware program you want to install in the right-side menu and click somewhere in the playing area to build it there. Remember that there's no path in this game and it's quite important to make sure the enemies will pass within the firing range of your turrets.